A recording-type optical disc is capable of holding a large amount of information and has a feature that it is a replaceable (i.e., swapable) medium. In reproduction of information recorded on the optical disc, a beam of laser light is focused onto an information recording side thereof, and light modulated by a record mark is reflected for detection. In recording of information on the optical disc, a laser beam having a power which is larger than a laser beam power used in reproduction is applied to the information recording side to form a record mark thermally.
Recording-type optical disc media are roughly classified into the following three types: (1) magneto-optical type, (2) phase-change type, and (3) pit-forming type. For rewritable recording, magneto-optical type discs are in widespread use, and for write-once recording, organic-pigment pit-forming type optical discs represented by recordable compact discs (CD-R) are popular.
To increase a storage density of the recording-type optical disc, precise control of recording power is required since smaller record marks must be formed with higher and higher precision, in higher and higher density. In an actual optical disc apparatus, however, even if an output level of a light source is kept constant, it is difficult to provide a required temperature distribution on the information recording side of the optical disc due to adverse effects, e.g., dynamic variations in ambient temperature, laser wavelength, beam spot distortion, etc.
Therefore, as disclosed in Japanese Non-examined Patent Publication 195713/1994, a technique called "trial writing" is employed in recording information on the recordable compact disc (CD-R). With this technique, trial writing is performed ti before user data is recorded, with the trial writing being conducted on a predetermined test area to determine an optimum level of recording power.
Furthering such discussion, in the FIG. 2a illustrated trial writing method, fine and coarse patterns are recorded alternately as shown. More specifically, a laser beam uses recording waveform 20 to create coarse 22 and fine 24 pits in a recording media, and upon reproduction, reproduced signals 26 and 28 are obtained from the coarse 22 and fine 24 pits, respectively. Using reproduced signals, a difference in an average level between fine and coarse patterns, i.e., an asymmetry value .DELTA.V (FIG. 2a) is detected, and a recording power level Po where the asymmetry value becomes approximately zero (middle example; FIG. 2b) is determined as an optimum recording condition. If the recording power level P is lower than Po (top example; FIG. 2b), .DELTA.V takes a negative value since the record mark is smaller than a specified shape. On the contrary, if the recording power level P is higher than Po (bottom example; FIG. 2b), .DELTA.V takes a positive value since the record mark is larger than the specified shape. Therefore, an optimum recording power level Po can be determined through detection of asymmetry .DELTA.V by changing the recording power in a proper range, and determining a power Po where asymmetry .DELTA.V=o. In this method, it is possible to attain a linear response as long as the width of a record mark is constant, even if the length thereof varies.
Description will now give further background and then tend toward discussion of problems occurring in application of the above-discussed "asymmetry detection" trial writing method while recording on phase-change optical discs. Since the information recorded on the phase-change optical disc is reproduced using a difference in reflectance between crystal and amorphous states of the media, the same type of reproducing circuit as for a CD-ROM may be used, i.e., the phase-change type of optical disc has an advantage of possible compatibility with the ROM type of optical disc.
As background on the phase-change optical disc, a record mark is formed as an amorphous state by melting a spot on a recording layer thereof with a laser beam and then quenching the spot. To erase the record mark, the amorphous state thereof is crystallized by irradiating the spot with laser heat having a temperature that is higher than a level of crystallization and lower than a melting point. If the quenching timing is delayed after melting in information recording, the spot is recrystallized. This phenomenon is called "recrystallization". Therefore, the shape of record mark is determined depending on spot cooling conditions as well as achieved temperature distribution. These are particularities of the phase-change optical disc recording mechanism, which are different from other mechanisms for recording such types of optical discs as magneto-optical discs.
In an example of examination of a phase-change optical disc, characteristics of an exemplary "asymmetry detection" trial writing method were measured using a GeSbTe phase-change material as a recording layer. A sample disc consisted of a plastic substrate having a diameter of 120 mm and a thickness of 0.6 mm, which had a lamination of a ZnS--SiO2 primary optical interference layer, GeSbTe recording layer, ZnS--SiO2 secondary optical interference layer, Al--Ti reflective layer, and UV protective layer. On the substrate, there were formed track grooves with a pitch of approx. 0.7 .mu.m for land group recording. A recording waveform having three recording levels Pw, Pe and Pb as shown in FIG. 3 was used, and a channel clock signal Tw was employed (where T is a predetermined channel bit length). For forming a record mark nTw, `n-1` Tw/2-width pulses were applied. For data modulation, an "8-16" modulation method was employed in which 1 Tw was approx. 0.2 .mu.m. The shortest mark length was 3 Tw, and the longest mark length was 14 Tw. A laser beam having a wavelength of 680 nm was emitted from a semiconductor laser source, and a beam spot for recording was formed by means of focusing through an objective lens having a numerical aperture value of 0.6. In measurement, a linear velocity of 6 m/s was used. A center value of a power margin Po in overwriting random signals on the sample disc was 10.5 mW in a case of Pw, and 3.8 mW in a case of Pe. A recording power level for trial writing was changed while maintaining a Pw-to-Pe ratio at 10.5 mW to 3.8 mW. A level of Pb was kept constant at 0.5 mW. Repetitive 3 Tw mark-space recording was made for fine patterning, and repetitive 8 Tw mark-space patterning was made for coarse patterning.
FIG. 4 shows a relationship between recording power and asymmetry .DELTA.V plotted in the measurement mentioned above, and a problem caused by recrystallization. On the axis of ordinate in this figure, the amount of asymmetry .DELTA.V was normalized with coarse pattern signal amplification. In a recording power range of 9 to 14 mW, the asymmetry .DELTA.V had a gradually increasing characteristic, with variation of up to 15% on the positive side and variation of just approx. 3% on the negative side. There was a tendency that the slope of asymmetry .DELTA.V was relatively gentle in a recording power range lower than Po. In the vicinity of the start point of recording, there occurred a phenomenon of code reversal.
These characteristics in the lower recording power range resulted from a problem owed to the above mentioned recrystallization in recording. More particularly, in comparison between coarse and fine patternings, a laser irradiation time in fine patterning is shorter than that in coarse patterning. Therefore, in fine patterning, the degree of thermal retention is smaller and heating and cooling are performed more rapidly, leading to a smaller extent of recrystallization. Since a difference in recrystallization between the coarse and fine patternings is larger in the vicinity of a recording threshold, the width of record mark in the fine patterning becomes thicker than that in the coarse patterning. The amount of asymmetry varies differently on the positive and negative sides and it cannot be determined definitely with respect a certain level of recording power, which means that complex processing procedures are required for determination of an optical power level Po using the "asymmetry detection" trial writing method.
Next, the following describes characteristics and problems related with a rewriting service life of the phase-change optical disc. As rewriting on the phase-change optical disc is repeated, the disc deteriorates gradually. Two of the most appreciable deterioration phenomena are; (1) fluidization of recording layer, and (2) change in reflectance. It is thought that the fluidization of a recording layer occurs due to thermal stress applied in the melted state of the recording layer at the time of recording. A change in reflectance, which is related with the phenomenon of recording layer fluidization, is thought to occur due to such causes induced by thermal stress as segregation in recording layer composition, penetration of interference layer materials, etc.
FIGS. 5a and 5b show examples of deterioration characteristics of phase-change optical discs used in an experimental examination. Referring to FIG. 5a, there is shown a graph indicating a relationship between length of record marks and degree of fluidization. In the examination, overwriting was performed 80,000 times continuously using a recording power Po. Each pattern in FIG. 5a indicates a repetitive pattern containing mark and space codes equally. At intervals of 50 bytes, each block consisting of 200 bytes was recorded.
As to fluidization, a length of a region where the initial signal amplitude decreased to less than 1/2 was measured at the beginning and end of each block. In FIG. 5a, a length of each fluidization region is indicated with respect to the beginning block. As can be seen from this figure, the length of a fluidization region was longer as the length of mark was shorter. For example, in a case of a 3 Tw mark, the fluidization region length thereof was more than double that of a 11 Tw mark.
Referring to FIG. 5b, there is shown a normalization plot indicating average quantities of reflected light from 3 Tw and 8 Tw patterns over repetitive writings, with respect to a 100% initial value level. As the number of rewriting operations increases, the average quantity level of reflected light decreases. In comparison between 3 Tw and 8 Tw patterns, the slope of the curve indicating a decrease in quantity of reflected light from 3 Tw patterns does not match that of 8 Tw patterns. This signifies that a rate of deterioration of the recording layer, as well as fluidization, depends on the length of mark. Since a difference in average quantity level of reflected light represents the amount of asymmetry, the plot in FIG. 5b reveals that the amount of asymmetry varies with the number of rewriting operations, even if the same level of power is applied. That is, if the number of rewriting operations is different between the test area for trial writing and areas for actually recording user data, it is impossible to set up a proper recording level of laser power.
As described above, it was found that the above-described trial writing method based on "asymmetry detection" is not suitable (i.e., is disadvantageous) for determination of an optimum recording power level on the phase-change optical disc because of the following reasons: (1) recrystallization and the differences in heating/cooling times between coarse and fine marks (pits), (2) fluidization, (3) improper linearity and indefinite determination characteristic in target point detection, and (4) dependency of recording layer deterioration on length of record mark.